The thermostat on the wall reads 21°C. The room feels like 26°C. Staff are complaining, productivity is dropping, and the facilities manager is staring at a controller that insists everything is fine. This mismatch between what the thermostat reports and what people actually experience is one of the most frustrating HVAC problems in commercial buildings — and it’s more common than you’d think.
The AC system is only as good as the temperature reading it’s working from. If the thermostat or sensor is wrong, the system targets the wrong temperature and the room is either too warm or too cold regardless of how well the AC unit itself is performing. Here are the most likely reasons the reading is off.
We see this issue regularly in Mayfair. A proactive PPM agreement is the best way to prevent this fault from recurring, and it means faster response times if you do need an commercial AC installation.
Thermostat in Direct Sunlight or Near a Heat Source
This is the most common cause and the easiest to overlook. A thermostat mounted on a wall that gets afternoon sun will read several degrees higher than the actual room temperature. The same happens when a thermostat is positioned near a radiator, above a piece of electrical equipment, next to a hot water pipe running through the wall, or beside a window with solar gain.
The result: the AC system overcools because the thermostat thinks the room is warmer than it is. Or in winter, the heating never kicks in because the thermostat is being warmed by an external heat source and thinks the room has already reached setpoint. Either way, the occupants are uncomfortable and the energy bills are wrong.
The fix is relocation. The thermostat needs to be on an interior wall, approximately 1.5 metres above floor level, away from direct sunlight, heat sources, draughts, and doorways. It sounds basic, but we regularly find thermostats installed in the worst possible locations — above photocopiers, next to south-facing windows, or directly beneath downlighters that pump heat onto the sensor.
Failed NTC Sensor
Most commercial HVAC thermostats use an NTC (Negative Temperature Coefficient) thermistor to measure temperature. These are generally reliable, but they do drift over time and they can fail. A failed NTC sensor might read a fixed offset — consistently 3°C low, for example — or it might give wildly erratic readings that bounce around with no relation to the actual room temperature.
An offset reading is particularly insidious because the system appears to work normally — it heats and cools in response to demand — but it’s always targeting the wrong temperature. A quick comparison with a calibrated handheld thermometer placed next to the thermostat will reveal any offset. If the NTC has failed, the sensor or the entire thermostat unit needs replacing.
Thermostat Needs Recalibration
Some commercial controllers and BMS (Building Management System) sensors allow field calibration — an offset value that compensates for sensor drift or installation position. If this calibration has been set incorrectly, accidentally changed, or never done in the first place, the reading will be off by whatever the offset error is.
Check the controller settings. Look for a calibration or offset parameter. Compare the displayed temperature with a known-accurate thermometer and adjust the offset accordingly. On BMS-connected sensors, this is usually a parameter in the controller software rather than a physical adjustment on the sensor.
Wireless Thermostat with Dead Batteries
Wireless thermostats are increasingly common in commercial fit-outs because they avoid the cost of running cable back to the controller. But they run on batteries, and when those batteries die or run low, the thermostat behaviour becomes erratic. It might display a temperature reading that’s frozen at the last value before the battery dropped, send intermittent signals to the controller, or stop communicating entirely.
The AC system may default to a fallback mode — either running continuously or shutting down — depending on how it’s programmed to handle a lost sensor. Check the battery level. Replace with fresh batteries. If the thermostat has been running on dying batteries for weeks, it may need to be re-paired with the controller.
Thermostat in a Different Zone
In larger offices, it’s not unusual to find that the thermostat controlling a particular AC unit is physically located in a different area than the space the unit serves. The thermostat might be in a corridor, a server room, or a partitioned section of open plan that was created after the original HVAC installation.
The thermostat reads the correct temperature — for where it is. But that bears no relationship to the temperature in the space people are actually trying to cool. The fix is either relocating the thermostat into the served space or adding a remote sensor in the correct zone and configuring the controller to read from that sensor instead.
We have written a separate guide on an AC system not cooling the building which covers a related failure mode worth checking. Similarly, our article on AC short cycling causes and fixes explains another common cause of the same underlying issue.
Dirty or Blocked Return Air Sensor on a Ducted System
Ducted AC systems often measure room temperature via a sensor in the return air path rather than a wall-mounted thermostat. If that return air grille is blocked — by furniture, boxes, or a notice board someone has stuck over it — the sensor reads stagnant air that doesn’t represent the room conditions. Dust build-up on the sensor itself can also insulate it from the airstream and cause inaccurate readings.
Check the return air path. Make sure the grille is clear and unobstructed. Clean the sensor if accessible. On concealed ducted units, the sensor may be inside the return air plenum — an engineer will need to access the ceiling void to inspect and clean it.
Optimal Thermostat Placement — Getting It Right
For anyone planning a new installation or relocating a thermostat, the rules are straightforward. Mount it on an interior wall at 1.5 metres above floor level. Keep it away from windows, exterior walls, direct sunlight, heat-generating equipment, supply air diffusers, and doorways. It should be in the occupied zone of the space it controls — not in a cupboard, corridor, or adjacent room.
In open-plan offices, a central position works best. In rooms with multiple AC units, each unit ideally needs its own sensor in the zone it serves. Getting placement right during installation avoids years of comfort complaints and wasted energy.
ADK Can Fix It
Temperature control problems in commercial buildings are rarely about the AC unit itself — they’re about what the system is being told. A thermostat reading wrong by even 2°C can make a space permanently uncomfortable and waste significant energy. ADK engineers diagnose and correct thermostat and sensor faults across London — recalibration, relocation, replacement, BMS reprogramming, whatever it takes to get the reading right and the room comfortable. Call us.